ATMOSPHERIC MOISTURE (Chapter 4)

Average January Vapor Pressure (mb )

Average July Vapor Pressure (mb )

Vapor Pressure, Saturation Vapor Pressure, and Temperature e = vapor pressure Pressure gauge es depends only on the temperature! Gauge records saturation vapor pressure for 20°C es = saturation vapor pressure

SATURATION VAPOR PRESSURE AND TEMPERATURE The vapor pressure of saturated air (the saturation vapor pressure) depends ONLY on TEMPERATURE! Page: 99 FIGURE 4.10 Saturation vapor pressure increases with increasing temperature. At a temperature of 10°C, the saturation vapor pressure is about 12 mb, whereas at 30°C it is about 42 mb. The inset illustrates that the saturation vapor pressure over water is greater than the saturation vapor pressure over ice. Why would

CONDENSATION AND TEMPERATURE Page: 96 FIGURE 4.5 Condensation is more likely to occur as the air cools. (a) In the warm air, fast-moving water vapor molecules tend to bounce away after colliding with nuclei. (b) In the cool air, slow-moving vapor molecules are more likely to join together on nuclei. The condensing of many billions of water molecules produces tiny liquid water droplets. As air cools, condensation becomes more likely.

Page: 97 FIGURE 4.6 The water vapor content (humidity) inside this air parcel can be expressed in a number of ways.

Absolute humidity varies as the air’s volume (or density) up Page: 97 FIGURE 4.7 With the same amount of water vapor in a parcel of air, an increase in volume decreases absolute humidity, whereas a decrease in volume increases absolute humidity. Absolute humidity varies as the air’s volume (or density) changes. Note that water vapor remains constant.

SPECIFIC HUMIDITY Page: 97 FIGURE 4.8 The specific humidity does not change as air rises and descends.

RELATIVE HUMIDITY Assume unsaturated air Figure 4.11 (a) At the same air temperature, an increase in the water vapor content of the air increases the relative humidity as the air approaches saturation. Assume unsaturated air Air is closer to being saturated, so RH increases

RELATIVE HUMIDITY Figure 4.11 (b) With the same water vapor content, an increase in air temperature causes a decrease in relative humidity as the air moves farther away from being saturated. Despite same water content (vapor pressure), RH lowers because of higher temperature and thus higher saturation vapor pressure.

SATURATION VAPOR PRESSURE AND TEMPERATURE It takes a lot more water vapor to saturate warm air than cold air. Page: 99 FIGURE 4.10 Saturation vapor pressure increases with increasing temperature. At a temperature of 10°C, the saturation vapor pressure is about 12 mb, whereas at 30°C it is about 42 mb. The inset illustrates that the saturation vapor pressure over water is greater than the saturation vapor pressure over ice. Why would

Constant temperature, increasing vapor pressure RH increases Page: 101 FIGURE 4.11 (a) At the same air temperature, an increase in the water vapor content of the air increases the relative humidity as the air approaches saturation. (b) With the same water vapor content, an increase in air temperature causes a decrease in relative humidity as the air moves farther away from being saturated. Increasing temperature, constant vapor pressure RH decreases

DIURNAL VARIATION OF RELATIVE HUMIDITY Page: 101 FIGURE 4.12 When the air is cool (morning), the relative humidity is high. When the air is warm (afternoon), the relative humidity is low. These conditions exist in clear weather when the air is calm or of constant wind speed. RH tends to be highest when it is coldest.

Phase changes occur at constant temperature. (pp. 34–36) LATENT HEAT Page: 35 FIGURE 2.3 Heat energy absorbed and released. Phase changes occur at constant temperature. (pp. 34–36)

Phases of Water Page: 95 FIGURE 4.3 The three states of matter: water as a gas, as a liquid, and as a solid.

Phases of Water ICE LIQUID WATER (note higher density)

DEW POINT TEMPERATURE AND SOURCE REGION Page: 105 FIGURE 4.19 Air from the Pacific Ocean is hot and dry over land, whereas air from the Gulf of Mexico is hot and muggy over land. For each city, T represents the air temperature, Td the dew point, and RH the relative humidity. (All data represent conditions during a July afternoon at 3 p.m. local time.)

JANUARY DEW POINT TEMPERATURE Page: 102 FIGURE 4.14 Average surface dew-point temperatures (°F) across the United States and Canada for January.

JULY DEW POINT TEMPERATURE Page: 103 FIGURE 4.15 Average surface dew-point temperatures across the United States and Canada (°F) for July.

DEW POINT TEMPERATURE January July Figure 4.13 Average surface dew-point temperatures (ºF) for (a) January and for (b) July. July

HYDROLOGIC CYCLE Page: 94 FIGURE 4.1 The hydrologic cycle.

Quick Summary—Humidity (chapter 4): Water exists in all three phases under normal conditions on Earth. Phase changes occur at a constant temperature, with latent heat being taken in or released. The vapor pressure of saturation depends only on air temperature. The easiest way to saturate air is to cool it, thus lowering the amount of vapor needed to saturate it. The numerous variables used to characterize humidity levels are all designed to suit different purposes.